Heavy oil activator and preparation method and use thereof

ABSTRACT

Disclosed are a heavy oil activator, a preparation method and use thereof. The heavy oil activator has a structural formula shown in Formula (1). In Formula (1), x, y, z, m, and n are respectively mass fractions of corresponding chain segments in the polymer, m being 0.75-0.85, y being 0.20-0.24, and x, z, and n all being 0.001-0.01; and p is a natural number between 2 and 9. The activating polymer provided has the effects of increasing viscosity of an aqueous phase while reducing viscosity of crude oil, and can serve both as a displacing agent and a viscosity reducer to realize integration of the two agents. The activating polymer can increase viscosity of an aqueous phase, has a displacement effect, can emulsify and disperse crude oil, and increase fluidity of a mixed phase; the dispersed crude oil continues to interact with crude oil that has not contacted the polymer, and the aqueous phase interacts with the mixed oil phase as well as new oil, by way of which continuous interactions, more crude oil can be constantly produced. The activating polymer has a relatively low interfacial tension and exhibits a certain degree of oil washing capability.

PRIORITY CLAIM

The present application is a continuing application of PCT PatentApplication No. PCT/CN2018/075360, filed Feb. 6, 2018, and claims thepriority of China Application No. 201710659444.X, filed Aug. 4, 2017,and China Application No. 201710659447.3, filed Aug. 4, 2017, theentirety of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a heavy oil activator, a preparationmethod and use thereof, and belongs to the field of oilfielddevelopment.

BACKGROUND OF THE INVENTION

Heavy oil is much more difficult to exploit than conventional crude oildue to its high content of resins and asphaltenes as well as its highviscosity. At present, conventional heavy oil production techniquesmainly include approaches such as viscosity reduction by heating,viscosity reduction by dilution, viscosity reduction by hydrothermalcracking, viscosity reduction by microbes, chemical viscosity reduction,etc. Principles of these approaches are to reduce the viscosity of heavyoil and increase its fluidity by raising the temperature of heavy oil,light crude oils of same structure dissolving each other, catalyticcracking of heavy oil under high-temperature steam, subjecting heavy oilto microbial degradation, and by a chemical viscosity reducer,respectively, to realize effective production of heavy oil.

However, current heavy oil production techniques all have certainlimitations. The method of viscosity reduction by heating (thermalproduction) is complicated in process, high in energy consumption, andhas problems such as sand production in oil layers, gas channeling, highoil production cost, etc. At the same time, ground conditions andreservoir conditions in thermal production are strictly limited byscreening standards of the thermal production method. For example, heavyoil reservoirs with relatively deep depth or thin thickness are notsuitable for thermal production. The dilution method requiresconsumption of thin oil resources, but the reserves of thin crude oilare limited and output thereof is showing a decreasing trend, and thereis bound to be the situation that no thin oil can be used for mixing.The method of viscosity reduction by hydrothermal cracking requires thatthe catalyst be a liquid or a nano-scale water-soluble suspension, behighly temperature-resistant and basically harmless to human bodies andthe environment, which makes it difficult to realize large-scalepromotion and application of the production technique of viscosityreduction by catalytic cracking in a short period of time. Limitationsof the method of viscosity reduction by microbes is that microbes areeasily destroyed under oil reservoir conditions of high temperature,high salinity and high content of heavy metal ions, and surfactants andbiopolymers produced by microbes may possibly cause sedimentationthemselves. Besides, conditions for cultivating microbes are difficultto grasp. Chemical viscosity reduction refers to a method in which acertain chemical agent is added to heavy oil to reduce the viscosity ofcrude oil by the action of the chemical agent. A chemical agentapplicable to any crude oils and capable of reducing the viscosity underany conditions has not yet been found. Therefore, it is only possible toadopt different chemical viscosity reduction measures for crude oilshaving different properties and for different oil well productionconditions. Commonly used viscosity reduction methods include aviscosity reduction technique of adding an oil-soluble viscosityreducer, and a viscosity reduction technique of adding an emulsifier.Although there are many formulations for emulsifying viscosity reducers,there are still many problems. For example, the liquid produced by usingthem is difficult to treat, and their selectivity to heavy oil is veryhigh because of the great difference in the composition of heavy oil.How does the composition of heavy oil affect the viscosity reductioneffect achieved by emulsification, and how does the structure of anemulsifying viscosity reducer be related to its properties. There arecurrently not many emulsifying viscosity reducers that can be used underhigh-temperature and high-salinity reservoir conditions. Even if thereare related reports in the literature, most of them are costly.

With the development of oilfield, it has become more and more difficultto achieve the goal of reducing viscosity and improving the overallbenefit of oilfield by conventional viscosity reduction techniquesalone. It is therefore of great practical significance to studyunconventional chemical viscosity reduction techniques suitable for thedevelopment mode of heavy oil reservoirs based on the conventionalchemical viscosity reduction techniques.

SUMMARY OF THE PRESENT INVENTION

The objective of the present invention is to provide a heavy oilactivator which can increase the viscosity of aqueous phase, and have adisplacement function. The heavy oil activator can emulsify and dispersecrude oil, and increase the fluidity of a mixed phase. Besides, thedispersed crude oil continues to interact with crude oil that has notcontacted the heavy oil activator, and the aqueous phase interacts notonly with the mixed oil phase but also with new oil. Productionefficiency at oilfields can thus be enhanced.

The activating water polymer provided by the present invention has astructural formula shown in Formula (1):

wherein, in Formula (1), x, y, z, m, and n are respectively massfractions of corresponding chain segments in the activating waterpolymer, m being 0.75-0.85, y being 0.20-0.24, and x, z, and n all being0.001-0.01; and

p is a natural number between 2 and 9.

In Formula (1), m is preferably 0.8, y is preferably 0.2, x ispreferably 0.005, z is preferably 0.003, and n is preferably 0.002.

The present invention further provides a preparation method of theactivating water polymer, which comprises the following steps:

subjecting acrylamide, a functional monomer I, a functional monomer II,and a functional monomer III to a free radical polymerization reactionin an inert atmosphere, in presence of an initiator, and under analkaline condition, to obtain the activating water polymer;

the functional monomer I has a structural formula as shown in Formula(2),

wherein in Formula (2), p is a natural number between 2 and 9;

the functional monomer II has a structural formula as shown in Formula(3).

the functional monomer III has a structural formula as shown in Formula(4);

In the above preparation method, the method is carried out according tothe following steps:

adding acrylamide, the functional monomer I, the functional monomer II,and the functional monomer III to water to obtain a reaction solution,adjusting a pH value of the reaction solution, and then introducingnitrogen into the reaction solution; lowering temperature of thereaction solution using an ice brine bath, and then adding the initiatorto carry out the free radical polymerization reaction; wherein heatreleased by the free radical polymerization reaction raises thetemperature of the reaction system, and after the temperature of thereaction system reaches a highest point, the free radical polymerizationreaction is continued for 2-6 hours.

In the above preparation method, the pH value is adjusted to 7-11 byusing sodium carbonate.

In the above preparation method, the temperature of the reactionsolution is lowered to 6-15° C. using the ice brine bath.

In the above preparation method, the highest temperature reached by thereaction system is 25-40° C.

In the above preparation method, adding amounts of the functionalmonomer I, the functional monomer II, and the functional monomer III bymass are respectively 0.02-0.2%, 0.02-0.4%, and 0.01-0.5% of an addingamount of acrylamide by mass, and may particularly be 0.04%, 0.04%, and0.03%, respectively, of the adding amount of acrylamide by mass.

In the system of the free radical polymerization reaction, a total massconcentration of the reactive monomers is 25-30%, the total massconcentration of the monomers referring to a sum total of acrylamide,the functional monomer I, the function monomer II, and the functionalmonomer III.

In the above preparation method, the initiator is a mixture of ammoniumpersulfate, potassium persulfate, sodium hydrogen sulfite, sodiumthiosulfate, azoisbutyramidine dihydrochloride, andazoisobutyrimidazoline dihydrochloride.

Adding amounts of ammonium persulfate, potassium persulfate, sodiumhydrogen sulfite, sodium thiosulfate, azoisbutyramidine dihydrochloride,and azoisobutyrimidazoline dihydrochloride by mass may respectively0.001-0.0045%, 0.001-0.005%, 0.001-0.006%, 0.001-0.005%, 0.001-0.006%,and 0.001-0.004% of an adding amount of acrylamide by mass, and mayparticularly be 0.002%, 0.002%, 0.003%, 0.002%, 0.003, and 0.002%,respectively, of the adding amount of acrylamide by mass.

In the reaction of the free radical polymerization reaction, a massvolume concentration of the initiator may be 45-60 mg/L.

In the above preparation method, the functional monomer I is preparedaccording to a method comprising the following steps:

(1) subjecting 1,3-bis(N,N-dimethyl)-2-propanol and a brominated alkaneto a quaternerization reaction to obtain an intermediate product,

wherein the brominated alkane has 2-9 carbon atoms; and

(2) subjecting the intermediate product and acryloyl chloride to anesterification reaction under an alkaline condition to obtain thefunctional monomer I.

In the above preparation method, in step (1), the quaternerizationreaction is carried out at a temperature of 65-70° C. for 12-48 hours,and ethanol is adopted as a solvent.

In the above method, in step (2), acryloyl chloride is slowly addeddropwise under ice water bath;

the alkaline condition is regulated and obtained by using triethylamine;

hydroquinone and dimethylaminopyridine are also added to the reactionsystem.

The activating polymer provided by the present invention has the effectsof increasing viscosity of an aqueous phase while reducing viscosity ofcrude oil, and can serve both as a displacing agent and a viscosityreducer to realize integration of the two agents. The activating polymercan increase viscosity of an aqueous phase, has a displacement effect,can emulsify and disperse crude oil, and increase fluidity of a mixedphase; the dispersed crude oil continues to interact with crude oil thathas not contacted the polymer, and the aqueous phase interacts with themixed oil phase as well as new oil, by way of which continuousinteractions, more crude oil can be constantly produced. The activatingpolymer of the present invention realizes a relatively low interfacialtension and exhibits a certain degree of oil washing capability. Theactivating polymer of the present invention has good profile controlability, and the dispersed emulsified oil per se has profound profilecontrol and displacement effects.

When the concentration of the activating polymer of the presentinvention is 400-3000 mg/L, the viscosity thereof is 20-40 mPa·s; theshear viscosity retention rate is 50-80%; the viscosity retention rateafter 90-day aging is 40-60%; the activating polymer has good saltresistance; the salinity is between 10000-20000 mg/L; the viscosityreducing rate of the flooding system of the activating water polymer isless than 5%; the oil-water interfacial tension is 0.1-1×10⁻¹ mN/m. Forheavy oil having a viscosity of below 1500 mPa·s, it has a viscosityreducing rate of 80-95%. The oil-to-water volume ratio between theactivating polymer and the heavy oil is less than 9:1, which basicallymeans formation of an oil-in-water emulsion.

The activating polymer of the present invention is suitable for use inan oilfield reservoir stratum having a crude oil viscosity of 1-1500mPa·s, and having a permeability difference of 0.1-10 D. The activatingpolymer can be used according to the following steps.

While the oilfield is being mined or when a water content of a liquidproduced under water flooding is 0-80%, an activating water displacingoil system is injected by an injection pump in a pressurized manner.Specifically, dry power of the activating polymer is first dissolvedwith injection water to prepare a high concentration mother solution,and then the mother solution is diluted to obtain the activating waterdisplacing oil system of a target concentration, and then the activatingwater displacing oil system is injected, by an injection pump in apressurized manner, from an injection well into a target oil layeraccording to a predetermined daily injection volume per single well.

The concentration of the mother solution is 4000-5000 mg/L, and thetarget concentration is 400-3000 mg/L.

The daily injection volume per single well is 200-3000 m³.

Compared with conventional polymers and high-viscosity polymers, thepolymer of the present invention can increase oil production by 10-20%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a nuclear magnetic spectra representative of structure of afunctional monomer I.

FIG. 2 is a nuclear magnetic spectra representative of an activatingpolymer.

FIG. 3 is an infrared spectra representative of the activating polymer.

FIG. 4 is a graph showing viscosity and concentration of an activatingwater displacing oil.

FIG. 5 is a graph showing long-term stability of the activating waterdisplacing oil.

FIG. 6 is a graph showing oil-water interfacial tension of theactivating water displacing oil.

FIG. 7 is a photomicrograph showing emulsification of crude oil by theactivating polymer.

FIG. 8 is a graph showing viscosity-reducing effects of differentconcentrations of activating polymer on heavy oil (data used for theviscosity test: record a relatively stable viscosity value of each ofmixed phases).

DETAILED DESCRIPTION OF THE EMBODIMENTS

Experimental methods used in the following examples are conventionalmethods unless otherwise specified.

Materials, reagents and the like used in the following examples arecommercially available unless otherwise specified.

Example 1

1. Preparation of an Activating Polymer

The activating polymer represented by Formula (1) was specificallyprepared by the following method.

First, functional monomer I was prepared by the following steps.

14.6 g (0.1 mol) of 1,3-bis(N,N-dimethyl)-2-propanol and 42.5 g (0.22mol) of bromooctane in ethanol solution were placed in a 250-mlthree-necked flask, stirred magnetically, and condensed under reflux at65-70° C. for a reaction for 24 hours. The resultant solution wassubjected to vacuum rotary evaporation at 50° C. to remove the solvent,followed by an addition of an appropriate amount of acetone, heated anddissolved. After the resultant solution was cooled, a white solid wasprecipitated with diethyl ether to obtain product I. The product wasrecrystallized three times with acetone-diethyl ether to give a whitesolid. The yield was greater than 95%. 26.6 g (0.05 mol) of product I inanhydrous chloroform solution was poured into a 250-ml three-neckedflask, followed by a dropwise addition of 6.06 g (0.06 mol) oftriethylamine in chloroform solution, and an addition of a small amountof hydroquinone and dimethylaminopyridine (DMAP), and stirred, followedby slowly adding 5.43 g (0.06 mol) of acryloyl chloride under ice waterbath. The resultant solution was allowed to stand at room temperatureovernight, stirred at 40° C. for 48 hours, and then subjected to vacuumrotary evaporation at room temperature to remove the solvent, followedby an addition of acetone and dissolved, and filtered to remove theinsoluble substance to obtain a filtrate. Diethyl ether was added to thefiltrate to precipitate a solid. The solid was repeatedly recrystallizedthree times with a mixed solvent of acetone and diethyl ether to give awhite functional monomer I of 14.6 g. The yield was 49.8%.

Structure of functional monomer I was represented using a nuclearmagnetic spectra (FIG. 1): ¹H NMR (400 MHz, CDCl₃): δ0.87 (6H, t,—CH₂—CH₃), 1.26-1.34 (20H, m, —(CH₂)₅—), 1.74-1.80 (4H, m, —N—CH₂—CH₂),3.42-3.50 (12H, d, N—(CH₃)₂), 3.56 (4H, m, N—CH₂—), 4.63-4.69 (2H, m,N—CH₂—CH), 4.87 (2H, d, N—CH₂—CH), 5.99 (1H, d, CH₂═CH—), 6.16-6.26 (1H,q, CH₂═CH—), 6.37 (1H, m, O—CH—), 6.89-6.93 (1H, d, CH₂═CH—).

Then, an activating polymer was prepared by the following steps.

According to a formulation of a polymerization reaction solution,acrylamide, functional monomer I, functional monomer II, and functionalmonomer III were added into a reaction kettle, and stirred until theywere completely dissolved. Adding amounts of functional monomer I,functional monomer II, and functional monomer III were 0.04 wt %, 0.04wt %, and 0.03 wt % of acrylamide, respectively, and a total amount ofmonomers in the reaction system was 25 wt %. The reaction solution wasadjusted with sodium carbonate to have a pH value of 10, and thennitrogen was introduced into the reaction solution for 60 minutes toremove oxygen from the reaction system. The reaction system was cooledto 6-15° C. using an ice brine bath, and then an initiator system wasadded under the protection of nitrogen. The initiator system was amixture of ammonium persulfate, potassium persulfate, sodium hydrogensulfite, sodium thiosulfate, azoisbutyramidine dihydrochloride andazoisobutyrimidazoline dihydrochloride. The components of the mixturewere respectively 0.002%, 0.002%, 0.003%, 0.002%, 0.003%, and 0.002% bymass of acrylamide. As reaction heat was continuously released from thereaction, the temperature of the reaction system rose. After thetemperature of the reaction system reached a highest point (25˜40° C.),the reaction was continued for 3 hours. After the reaction wascompleted, the gel product in the reaction kettle was granulated, dried,and sieved to obtain a granulated product of an activating polymer oildisplacing agent.

A nuclear magnetic spectra and an infrared spectra representative of theactivating polymer prepared in this example are shown in FIG. 2 and FIG.3, respectively. It can be confirmed that the prepared polymer has acorrect structure.

The structural formula of the activating polymer prepared in thisexample is represented by Formula (1), where m is 0.8, y is 0.2, x is0.005, z is 0.003, and n is 0.002.

2. Application Performance of Activating Water Displacing Oil

A solution was prepared according to the “Technical Criterions ofPolymer for Oil Displacement SY/T 5862-2008”. Dry powder of theactivating polymer was dissolved quickly with injection water to preparea high concentration mother solution (5000 mg/L). The mother solutionwas then diluted to obtain an activating water displacing oil with atarget concentration. The injection water was simulated mineralizedwater of an oilfield in Bohai region, and ion composition thereof was:Na⁺+K⁺ 3091.96, Ca²⁺ 276.17, Mg²⁺ 158.68, CO₃ ²⁻ 14.21, HCO₃ ⁻ 311.48,SO₄ ²⁻ 85.29, Cl⁻ 5436.34. In the experiment, the crude oil was crudeoil of an oilfield in Bohai region, and viscosity of the formation was400 mPa·s (at 65° C.).

(1) Viscosity-Concentration Relationship

BROOKFIELD DV-III was used for viscosity testing before and aftershearing. Shearing was performed using a Waring blender. Standards forthe shearing were that the shearing strength was set in the first gear,and that the shearing was performed for 20 seconds. The rotation speedwas set to 6 r/min (7.34 s⁻¹) for an apparent viscosity test. The testtemperature was 65° C. A viscosity-concentration curve of the activatingwater displacing oil was obtained (FIG. 4). An activating waterdisplacing oil of 1500 mg/L was prepared, and was stirred for 20 secondswith a Waring blender in the first gear. The apparent viscosity afterstirring was measured at 65° C. and a shearing rate of 7.34 s⁻¹. Theactivating water displacing oil was placed in a conical flask, intowhich nitrogen was then introduced to remove oxygen. The flask wassealed with a plug and placed in a 65° C. incubator. The viscosity ofthe activating water displacing oil was measured at different timepoints (FIG. 5). As can be noted from the figure, the viscosity is 28mPa·s when the concentration is 1500 mg/L; the viscosity after shearingis 16.8 mPa·s; and the viscosity retention rate after shearing is 60%.After 90 days of aging, the viscosity is 13.5 mPa·s, and the viscosityretention rate after aging is 48.2%.

(2) Oil-Water Interfacial Tension

The activating polymer can significantly reduce the oil-waterinterfacial tension, as shown in FIG. 6, to about 10⁻¹ mN/m. The abilityof the activating polymer to reduce the oil-water interfacial tensionindicates that it has a good interfacial activity.

(3) Dispersion Performance

A fluorescence microscope was used to observe the dispersion morphologyof different concentrations of the activating polymer and a smallmolecule viscosity reducer (sodium alkyl sulfonate) after theirinteraction with heavy oil. The oil-water ratio was 1:1. Micrographs ofemulsified crude oil are shown in FIG. 7.

As can be noted from FIG. 7, the small molecule surfactant, at a lowconcentration, has a weak ability to emulsify heavy oil. Compared withthe small molecule viscosity reducer, the activating polymer solution,at a concentration of as low as 400 mg/L, can significantly emulsify anddisperse heavy oil. When the concentration of the activating polymersolution is higher, emulsified and dispersed oil droplets have smallerparticle sizes. The micro-dispersed phase has a very wide sizedistribution range from a few hundred nanometers (the lower limit thatcan be observed) to a few dozen micrometers, and a small number ofmicro-phases have larger sizes.

(4) Viscosity Reducing Performance

Different concentrations of activating polymer solutions were prepared.The heavy oil to be tested and the activating polymer solution weremixed at a ratio of oil to water of 1:1, and kept at a constanttemperature of 65° C. for 1 hour. The resultant mixture was mechanicallystirred for 1 minute (600 r/min). The viscosity test was quickly carriedout at 65° C. using BROOKFIELD DV-III. Results are shown in FIG. 8.

It can be noted from FIG. 8 that the activating polymer has a goodviscosity reducing effect on heavy oils of different viscosities, andthe activating polymer solution at a concentration as low as 500 mg/Lhas a viscosity reducing rate for heavy oil of more than 50%. When theconcentration of the activating polymer is 1000 mg/L, its viscosityreducing rate exceeds 90%. However, when the concentration of theactivating polymer is increased to 1500 mg/L, its viscosity reducingrate is decreased slightly. The viscosity reduction of heavy oil by theactivating polymer is in essence viscosity reduction by emulsification.

3. Oilfield On-Site Example of Enhanced Oil Production by ActivatingWater Displacing Oil

An inland oilfield in China was selected as a target oilfield to carryout on-site test of enhanced oil production by activating waterflooding. Steps are as follows. First, dry powder of the activatingpolymer was quickly dissolved with injection water to prepare a highconcentration mother solution. The mother solution was diluted to obtainan activating water displacing oil system of a target concentration. Theactivating water displacing oil system was then injected, by aninjection pump in a pressurized manner, from an injection well into atarget oil layer according to a predetermined daily injection volume persingle well. The injection volume per single well was 2000 m³/d, and thepolymer concentration was 400-800 mg/L.

Conditions of a test block are shown in Table 1.

TABLE 1 Conditions of the Test Block Well spacing 100 m Injection well 3injection wells (4-4/6/12 well) Well pattern Inverted seven-spotUnderground crude oil viscosity 1170 mPa · s on average Asphaltenecontent 39.10% Permeability (54.3-800.7) × 10⁻³D Oil layer thickness5-21 m Temperature 35° C. Salinity of formation water 24100-33000 mg/LCalcium and magnesium ions 96.4-163 mg/L, 88.4-120 mg/L Injection time2011.8 Injection concentration 800 mg/L, 400 mg/L

The activating polymer for heavy oil achieved significant oil-increasingeffect at the tested oilfield. With respect to the oil productionincrease, the monthly oil production volume was 240 cubic meters beforethe injection of activator, and after the injection of activator, themonthly oil production was 1200 cubic meters in October 2011, reaching aproduction peak. By Oct. 14, 2013, the cumulative oil productionincrease was 7982 cubic meters, and each ton of the activator increasedthe oil production by 106 cubic meters.

INDUSTRIAL APPLICATION

The present invention has the following beneficial effects.

The activating polymer provided by the present invention has effects ofsimultaneously increasing the viscosity of the aqueous phase andreducing the viscosity of the crude oil, and can serve both as adisplacing agent and a viscosity reducer to realize the integration ofthe two agents. The activating polymer can increase the viscosity of theaqueous phase, has a displacement effect, can emulsify and disperse thecrude oil, and increase the fluidity of the mixed phase; the dispersedcrude oil continues to interact with the crude oil that has not yetcontacted the agent, and meanwhile the aqueous phase interacts not onlywith the mixed oil phase but also with new oil, by way of whichcontinuous interactions, more crude oil can be constantly produced. Theactivating polymer of the present invention has a low interfacialtension and exhibits a certain degree of oil washing capability. Theactivated polymer of the invention has good profile control ability, andthe dispersed emulsified oil has the function of deep profile controland displacement.

The invention claimed is:
 1. A polymer of dispersing agent having astructural formula shown in Formula (1):

wherein, in Formula (1), x, y, z, m, and n are respectively massfractions of corresponding chain segments in the polymer, m being0.75-0.85, y being 0.20-0.24, and x, z, and n all being 0.001-0.01; andp is a natural number between 2 and
 9. 2. A preparation method of thepolymer according to claim 1, comprising the following steps: subjectingacrylamide, a functional monomer I, a functional monomer II, and afunctional monomer III to a free radical polymerization reaction in aninert atmosphere, in presence of an initiator, and under an alkalinecondition, to obtain the polymer, wherein the functional monomer I has astructural formula as shown in Formula (2),

wherein in Formula (2), p is a natural number between 2 and 9; thefunctional monomer II has a structural formula as shown in Formula (3);and

the functional monomer III has a structural formula as shown in Formula(4);


3. The preparation method according to claim 2, wherein the method iscarried out according to the following steps: adding acrylamide, thefunctional monomer I, the functional monomer II, and the functionalmonomer III to water to obtain a reaction solution, adjusting a pH valueof the reaction solution, and then introducing nitrogen into thereaction solution; lowering temperature of the reaction solution usingan ice brine bath, and then adding the initiator to carry out the freeradical polymerization reaction; wherein heat released by the freeradical polymerization reaction raises the temperature of the reactionsystem, and after the temperature of the reaction system reaches ahighest point, the free radical polymerization reaction is continued for2-6 hours.
 4. The preparation method according to claim 3, wherein, thepH value is adjusted to 7-11 by using sodium carbonate; the temperatureof the reaction solution is lowered to 6-15° C. using the ice brinebath; and the highest temperature reached by the reaction system is25-40° C.
 5. The preparation method according to claim 2, wherein,adding amounts of the functional monomer I, the functional monomer II,and the functional monomer III by mass are respectively 0.02-0.2%,0.02-0.4%, and 0.01-0.5% of an adding amount of acrylamide by mass; andin the system of the free radical polymerization reaction, a total massconcentration of the reactive monomers is 25-30%, the total massconcentration of the monomers referring to a sum total of acrylamide,the functional monomer I, the function monomer II, and the functionalmonomer III.
 6. The preparation method according to claim 2, wherein,the initiator is a mixture of ammonium persulfate, potassium persulfate,sodium hydrogen sulfite, sodium thiosulfate, azoisbutyramidinedihydrochloride, and azoisobutyrimidazoline dihydrochloride; addingamounts of ammonium persulfate, potassium persulfate, sodium hydrogensulfite, sodium thiosulfate, azoisbutyramidine dihydrochloride, andazoisobutyrimidazoline dihydrochloride by mass are respectively0.001-0.0045%, 0.001-0.005%, 0.001-0.006%, 0.001-0.005%, 0.001-0.006%,and 0.001-0.004% of an adding amount of acrylamide by mass; and in thesystem of the free radical polymerization reaction, a mass volumeconcentration of the initiator is 45-60 mg/L.
 7. The preparation methodaccording to claim 2, wherein, the functional monomer I is preparedaccording to a method comprising the following steps: (1) subjecting1,3-bis(N,N-dimethyl)-2-propanol and a brominated alkane to aquaternerization reaction to obtain an intermediate product, wherein thebrominated alkane has 2-9 carbon atoms; and (2) subjecting theintermediate product and acryloyl chloride to an esterification reactionunder an alkaline condition to obtain the functional monomer I.
 8. Thepreparation method according to claim 7, wherein, in step (1), thequaternerization reaction is carried out at a temperature of 65-70° C.for 12-48 hours; in step (2), the alkane condition is regulated andobtained by using triethylamine; and hydroquinone anddimethylaminopyridine are also added to the reaction system.
 9. Apolymeric displacing agent, having an active ingredient which is thepolymer of claim
 1. 10. A method for improving oil production at anoilfield, comprising the following steps: injecting an oil displacementsystem while the oilfield is being mined or when a water content of aliquid produced under water flooding is 0-80%, wherein the oildisplacement system is an aqueous solution of the polymer of claim 1.11. The method according to claim 10, wherein, an oil reservoir of theoilfield is an oilfield reservoir stratum having a crude oil viscosityof 1-1500 mPa·s, and having a permeability difference of 0.1-10 D. 12.The method according to claim 10, wherein, the oil displacement systemis injected by an injection pump in a pressurized manner specificallyaccording to the following steps: preparing the polymer first into amother solution using injection water, and then diluting the mothersolution into the oil displacement system of a target concentration, andthen injecting the oil displacement system from an injection well to atarget oil layer by the injection pump in a pressurized manner.
 13. Themethod according to claim 12, wherein, the mother solution has aconcentration of 4000-5000 mg/L, and in the oil displacement system, thepolymer has a concentration of 400-3000 mg/L.